Several sources familiar with behind-the-scenes maneuvring say a number of other Democratic senators signed up for more sanctions had privately recoiled from a damaging vote against their own president.

According to some counts in recent weeks, the measure had 59 likely votes, including 16 Democrats, and was even approaching a two-thirds veto-proof majority in the 100-seat Senate.

But latest developments appear to have checked that momentum.

"I am strongly supporting the bill but I think a vote is unnecessary right now as long as there's visible and meaningful progress" in the Iran negotiations, Senator Richard Blumenthal told AFP, after expressing reservations earlier this month.

Democratic Senator Chris Coons made a similar declaration at a post-State of the Union event hosted by Politico.

"I did not sign it with the intention that it would ever be voted upon or used upon while we're negotiating," Manchin told MSNBC television.

"I signed it because I wanted to make sure the president had a hammer if he needed it and showed him how determined we were to do it and use it if we had to."

The White House mounted an intense campaign against a bill it feared would undermine Tehran's negotiators with conservatives back home or prompt them to ditch diplomacy.

Obama aides infuriated pro-sanctions senators by warning the measure could box America into a march to war to halt Tehran's nuclear program if diplomacy died.

The campaign included a letter to Reid from Democratic committee chairs urging a vote be put off.

Another letter was orchestrated from a group of distinguished foreign policy experts.

Multi-faith groups weighed in and coordinated calls from constituents backing Obama on nuclear diplomacy poured into offices of key Democrats.

The campaign appears for now to have overpowered the pro-sanctions push by hawkish senators and the Israel lobby, whose doubts on the Iran nuclear deal mirror those of Israeli Prime Minister Benjamin Netanyahu.

Senator Johnny Isakson, a Republican co-sponsor of the legislation, said: "It looks like we're kind of frozen in place."

Those behind the anti-sanctions campaign though privately concede they may have won a battle, not a war.

'A crucial victory'

The push for new sanctions will flare again ahead of the American Israel Public Affairs Committee's (AIPAC) annual conference in March, which Netanyahu is expected to address.

It could also recur if the talks with Iran on a final pact extend past the six-month window set by the interim deal.

But for now, groups that supported the push against sanctions are jubilant.

"This is a major victory, a crucial victory for the American public who don't want to see a war," said Kate Gould of the Friends Committee on National Legislation.

But she warned: "There'll be other efforts to try and sabotage the process."

The liberal pro-Israel lobby group J Street played a major role in the anti-sanctions push.

Vice president Alan Elsner said the group "continues to work hard to persuade lawmakers not to take action that risks sabotaging the negotiations with Iran.

"We're happy that more and more senators are seeing the logic of our argument," he added.